Pertussis toxin exerts its effects on mammalian cells by ADP-ribosylating regulatory proteins involved in signal transduction. The toxin comprises two components, an enzymatically active A subunit and a B oligomer needed for binding to the eukaryotic cell. Studies have been initiated to examine the mechanism of entry of pertussis toxin to eukaryotic cells. Unlike diphtheria toxin, pertussis toxin does not require an acidic environment (such as would be found in the endosome) to enter cells. Cytochalesins do not inhibit the ability of pertussis toxin to ADP-ribosylate its substrate in cells suggesting that this toxin may not enter cells by an endocytic mechanism. Currently, studies are underway to determine whether pertussis toxin might enter cells by penetrating the plasma membrane directly. The pathway of secretion of pertussis toxin from B. pertussis is also being examined. A mutant of B. pertussis has been isolated which is defective in secretion of this toxin. The lesion is currently being identified using both biochemical and genetic techniques. We have examined the immunogenicity of the pertussis toxin molecule. The abilities of the B oligomer and a holotoxin-like molecule composed of the B oligomer and a genetically-inactivated S1 subunit to protect mice against aerosol challenge with B. pertussis were compared. We found that the B oligomer contributes significantly to the protection afforded by the holotoxin-like molecule. A B oligomer was also assembled from individual subunits produced in E. coli using recombinant DNA techniques. This recombinant B oligomer contained each of the B subunit, had mitogenic activity characteristic of the B oligomer and protected mice against challenge with B. pertussis.